Dissected Fly Brain (IMAGE) Penn State Caption The dissected fly brain is stained for tyrosine hydroxylase immunoreactivity to visualize dopamine neurons. Tyrosine hydroxylase is a rate-limiting enzyme for dopamine biosynthesis and is present in dopamine neurons. Dopamine neurons comprise only about 0.1 percent of all the neurons in the fly brain. Credit Young-Cho Kim, Penn State Usage Restrictions None License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.